Carrier signals on power transmission lines have long been used by electric utilities as a means of communication between power stations. The radio frequency (RF) carrier is generally coupled to and from the high voltage line by series capacitors which block the 60 Hz high tension voltage and pass the high frequency low voltage carrier signal. In such applications, the carrier communication system represents but a minuscule fraction of the total investment of the utility so that there has been no great need for economy in equipment design.
More recently, in large part as a result of the general desire for economy in utilization of energy, the need for centralized remote control and night-time dimming of street lights has developed. City street lights are frequently energized in groups of up to 50 on 230 volt, 60 Hz A.C. power lines. One or more such lines may radiate from local controller boxes scattered throughout the city. The main breaker in the controller box is operated, either by clockwork or by remote control, to turn the lights on or off. For dimming lights such as high pressure sodium vapor or metal halide lamps, it is necessary to connect additional reactance into the lamp circuit at each individual luminaire. It has been proposed to do this by generating and coupling an RF carrier signal or tone into the power line at the controller box to cause the operation of a relay switching the dimming reactance into circuit in each individual luminaire.